Technical Field
Methods and example embodiments described herein are generally directed to interconnect architecture, and more specifically, to network on chip system interconnect architecture.
Related Art
The number of components on a chip is rapidly growing due to increasing levels of integration, system complexity and shrinking transistor geometry. Complex System-on-Chips (SoCs) may involve a variety of components e.g., processor cores, DSPs, hardware accelerators, memory and I/O, while Chip Multi-Processors (CMPs) may involve a large number of homogenous processor cores, memory and I/O subsystems. In both systems the on-chip interconnect plays a key role in providing high-performance communication between the various components. Due to scalability limitations of traditional buses and crossbar based interconnects, Network-on-Chip (NoC) has emerged as a paradigm to interconnect a large number of components on the chip.
NoC is a global shared communication infrastructure made up of several routing nodes interconnected with each other using point-to-point physical links. Messages are injected by the source component and are routed from the source components to the destination over multiple intermediate nodes and physical links. The destination component then ejects the message and provides it to the destination component. For the remainder of the document, terms ‘components’, ‘blocks’ hosts' or ‘cores’ will be used interchangeably to refer to the various system components which are interconnected using a NoC. Terms ‘routers’ and ‘nodes’ will also be used interchangeably. Without loss of generalization, the system with multiple interconnected components will itself be referred to as ‘multi-core system’.
There are several possible topologies in which the routers can connect to one another to create the system network. Bi-directional rings (as shown in FIG. 1(a)) and 2-D mesh (as shown in FIG. 1(b)) are examples of topologies in the related art.
Packets are message transport units for intercommunication between various components. Routing involves identifying a path which is a set of routers and physical links of the network over which packets are sent from a source to a destination. Components are connected to one or multiple ports of one or multiple routers; with each such port having a unique ID. Packets carry the destination's router and port ID for use by the intermediate routers to route the packet to the destination component.
Examples of routing techniques include deterministic routing, which involves choosing the same path from A to B for every packet. This form of routing is oblivious of the state of the network and does not load balance across path diversities which might exist in the underlying network. However, such deterministic routing may be simple to implement in hardware, maintains packet ordering and may be easy to make free of network level deadlocks. Shortest path routing minimizes the latency as it reduces the number of hops from the source to destination. For this reason, the shortest path is also the lowest power path for communication between the two components. Dimension-order routing is a form of deterministic shortest path routing in 2D mesh networks.
FIG. 2 illustrates an example of XY routing in a two dimensional mesh. More specifically, FIG. 2 illustrates XY routing from node ‘34’ to node ‘00’. In the example of FIG. 2, each component is connected to only one port of one router. A packet is first routed in the X dimension till the packet reaches node ‘04’ where the x dimension is same as destination. The packet is next routed in the Y dimension until the packet reaches the destination node.
Source routing and routing using tables are other routing options used in NoC. Adaptive routing can dynamically change the path taken between two points on the network based on the state of the network. This form of routing may be complex to analyze and implement and is therefore rarely used in practice.
A NoC may contain multiple physical networks. Over each physical network, there may exist multiple virtual networks, wherein different message types are transmitted over different virtual networks. In this case, at each physical link or channel, there are multiple virtual channels; each virtual channel may have dedicated buffers at both end points. In any given clock cycle, only one virtual channel can transmit data on the physical channel.
NoC interconnects often employ wormhole routing, wherein, a large message or packet is broken into small pieces known as flits (also referred to as flow control digits). The first flit is the header flit which holds information about this packet's route and key message level info along with payload data and sets up the routing behavior for all subsequent flits associated with the message. Zero or more body flits follows the head flit, containing the remaining payload of data. The final flit is tail flit which in addition to containing the last payload also performs some bookkeeping to close the connection for the message. In wormhole flow control, virtual channels are often implemented.
The physical channels are time sliced into a number of independent logical channels called virtual channels (VCs). VCs provide multiple independent paths to route packets, however they are time-multiplexed on the physical channels. A virtual channel holds the state needed to coordinate the handling of the flits of a packet over a channel. At a minimum, this state identifies the output channel of the current node for the next hop of the route and the state of the virtual channel (idle, waiting for resources, or active). The virtual channel may also include pointers to the flits of the packet that are buffered on the current node and the number of flit buffers available on the next node.
The term “wormhole” plays on the way messages are transmitted over the channels: the output port at the next router can be so short that received data can be translated in the head flit before the full message arrives. This allows the router to quickly set up the route upon arrival of the head flit and then opt out from the rest of the conversation. Since a message is transmitted flit by flit, the message may occupy several flit buffers along its path at different routers, creating a worm-like image.
Based upon the traffic between various end points, and the routes and physical networks that are used for various messages, different physical channels of the NoC interconnect may experience different levels of load and congestion. The capacity of various physical channels of a NoC interconnect is determined by the width of the channel (number of physical wires) and the clock frequency at which it is operating. Various channels of the NoC may operate at different clock frequencies. However, all channels are equal in width or number of physical wires. This width can be determined based on the most loaded channel and the clock frequency of various channels.